The present invention relates to a method for controlling a movable lens of an image forming apparatus, such as a copying machine so as to stop the lens at a home position and so as to move the lens from a present position to a specified position.
Recently, many image forming apparatuses such as copying machines have been provided in which image forming magnification is changeable. In some of such apparatuses, image forming magnification is changed stepwise or continuously by moving a lens, such as a zoom lens, a fixed focal length lens and the like.
In such image forming apparatus having a movable lens, lens position is corrected to a home position (life-size position) when turning on a power switch. For example, when the power switch is turned off in an enlargement mode, the lens remains at a position of the enlargement mode. In order to reset the apparatus in a life-size mode when the power switch is turned on, it is necessary to return the lens to the home position.
As a method for correcting a lens position to a home position more precisely, Japanese Unexamined Patent Publication No. 60-513 shows one in which irrespective of a lens being in a position of the enlargement mode or reduction mode, the lens position is corrected to a home position by moving the lens in one direction at all times, whereby the irregularity in lens stop positions is reduced which is caused by backrush or the friction of gears.
However, conventional methods including the above mentioned method execute access of a lens to a home position at the same speed as the speed at which usual movement of the lens is executed and is considerably high. Accordingly, it is liable to occur that the lens passes the home position during a time that a microcomputer or the like detects the home position arrival of the lens and then stops the lens, which consequently causes a difference between the home position and the actual lens stop position.
It will be seen that the above problem can be eliminated by reducing the moving speed of the lens in correcting the lens position to the home position. However, the reduction of the moving speed of the lens disadvantageously causes vibration of the lens due to resonance and the like. If the resonance is prevented by further reducing the moving speed of the lens, a waste of time will be involved. Especially, when there is a long distance between a present position of the lens and the home position, a very long time is required to move the lens to the home position.
As methods for moving a lens from a present position to a specified position, for example, there have been a method in which a lens is moved from a present position directly to a specified position (see Japanese Unexamined Patent Publication No. 60-114,847), and another method in which a lens is firstly returned to a home position and then moved to a specified position.
However, it will been seen that a position error inevitably occurs when actually moving the lens. Accordingly, in the abovementioned former method in which a lens is directly moved to a specified position, the positional error is accumulated by repeating the movement of the lens. Consequently, it is difficult to precisely control the lens position. On the other hand, in the latter method in which a lens is once returned to a home position and then moved to a specified position, such a defect as abovementioned can be eliminated. However, if a present position of a lens is near the specified position and at the same time these two positions are far from a home position, it takes much time to move the lens to the specified position. Accordingly, rapid control of lens cannot be achieved.